As mentioned before is is possible to have jobs or shifts without a public
registration. To let some people register for them you can create a link and
sent it to them.
Just create a new link and select some shifts. You can change the shifts also
after the link was sent. Of course it is also possible to include a publicly
visible job into a link.
The usage field is just for internal use to remember why a link was generated.
Links can only be created by admins, not by job admins.